KVM: guest_memfd: limit overzealous WARN

Because kvm_gmem_get_pfn() is called from the page fault path without
any of the slots_lock, filemap lock or mmu_lock taken, it is
possible for it to race with kvm_gmem_unbind().  This is not a
problem, as any PTE that is installed temporarily will be zapped
before the guest has the occasion to run.

However, it is not possible to have a complete unbind+bind
racing with the page fault, because deleting the memslot
will call synchronize_srcu_expedited() and wait for the
page fault to be resolved.  Thus, we can still warn if
the file is there and is not the one we expect.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>